Gyroscopes have found application in the sensing of angular rotation rate. The design and packaging of gyroscopes has evolved as new applications for their rate-sensing capabilities have been realized. For instance, gyroscopes have been utilized in airplane navigation systems, weapon navigation systems, and boat stabilization systems. More recently, gyroscopes have been utilized in applications such as stabilizing a camera lens and in providing real time feedback for interactive game consoles indicating when a control device has been moved.
Inertial rate gyroscopes include vibrating elements referred to herein as “gyro resonators.” These gyro resonators may take on one of many forms, including tuning fork, cylinder and planar ring structures. Many inertial rate gyroscopes utilize Coriolis forces to detect the angular rate of rotation of the gyro resonator about a sensitive axis. Inertial rate gyroscopes may be constructed from a variety of materials, including but not limited to piezoelectric, ceramic and quartz.
In some applications (e.g. aeronautics), the gyroscope may be subject to a range of operating temperatures. Temperature may affect the vibrational characteristics of the resonating element, which in turn may cause a change in the zero bias and scale factor of the gyroscope. The changes in bias and scale factor are herein collectively referred to as “temperature drift” which may be manifested as an error in the detected magnitude of angular rotation.
One solution for handling temperature drift is to utilize a temperature sensing device such as a thermistor or thermocouple that senses the temperature of the gyro resonator. Knowing the temperature of the gyro resonator enables some correction of the effects of temperature drift. Unfortunately, the temperature sensing device may significantly lead or lag the change in temperature of the gyro resonator, causing a transient error in the temperature determination. Moreover, the presence and configuration of the temperature sensing device may load the temperature measurement in a way that cannot be simulated during calibration, leading to a potential for steady state error in the temperature determination. Such transient and steady state errors in the temperature determination may lead to insufficient precision in the detection of rotational rates.
Installation of the temperature measuring device into the gyroscope assembly may increase the complexity and quality assurance requirements in the manufacture of the gyro resonator. For instance, the welds of the thermistor mounts may require inspection and testing to ensure connectivity to the resonating body for temperature detection. Even welds of highest quality may introduce asymmetries in the structure that affect the propagation of the vibration pattern of the gyro resonator in operation. The heightened complexity and quality assurance requirements may increase costs and reduce manufacturing output.
For silicon ring gyro resonators, the technique of inferring temperature from the resonant frequency of the gyro resonator, as well as from secondary indicia such as drive voltage level and quadrature sense signal level, is known. U.S. Pat. No. 7,120,548 to Malvern et al. (Malvern) discloses a method for implementing bias and scale factor corrections utilizing measurements of the resonant frequency from the silicon ring resonator. The technique is disclosed as being applicable to silicon ring gyro resonators having a substantially linear variation in resonant frequency with temperature of −0.4 Hz/Celsius in the vicinity of −40 Celsius. Such an approach eliminates the need for installation of the temperature measuring device and attendant quality assurance complexities.
However, the use of a silicon ring gyro resonator does not eliminate the effects of lead or lag in the sensing of temperature. The gyro resonator of a silicon ring gyroscope comprises a continuous silicon ring suspended from a support structure on thin silicon filaments. Drive and sense components, typically magnetic or capacitive in nature, are operatively coupled to the filaments and ring. In steady state operation, the temperature of the silicon ring is typically is elevated from the magnetic or capacitive drive/sense components due to flexure heating (i.e. dissipation of vibrational energy). The magnetic or capacitive drive/sense components respond more quickly to external temperature changes because they are more closely coupled to the external case of the gyroscope package than the silicon ring, which is isolated by the thin silicon filaments. Accordingly, with silicon ring resonators, there often remains a lead or lag in the ring temperature with respect to the magnetic or capacitive drive/sense components in response to external temperature changes. Malvern characterizes this lead or lag as an “apparent hysteresis” in the time domain. Malvern further discloses a method for correction that implements a power series expansion utilizing the resonant frequency of the gyro resonator, the drive voltage required to maintain a fixed vibration amplitude at the antinode of the oscillation pattern, and quadrature sense signal levels to correct for the effects of lead or lag on the bias.
In addition, many silicon ring gyro resonators have limited life cycles and durability issues. While the ring and filaments are made of silicon, the support structure is typically a composite structure of a glass or quartz material. As such, silicon ring gyro resonators are prone to failure due to delamination between the silicon and the glass or other dissimilar components. Fatigue of the thin silicon filaments is also a frequent mode of failure.
A device and method that can effectively compensate for the bias and scale factor errors associated with temperature drift while reducing the complexities associated with sensing the temperature of the gyro resonator in a more durable configuration would be welcome.